Conventionally, it is usual to connect two electrical devices (such as chip and substrate/film) using conductive bumps by flip-chip bonding or inner lead bonding. It is well known that the bumps on an integrated circuit chip are manufactured while the chip is still in wafer form. There are UBM (Under Bump Metallurgy) pads between the bumps and the bonding pads of the wafer configured to be a bonding interface for enhancing the bonding efficiency between the bonding pads and the bumps.
One of the well known bumping techniques is that the process of forming UBM pads is separated from the process of forming bumps. Normally the bumps are manufactured after completing UBM pads on bonding pads of a wafer.
Another prior art method for making UBM pads and bumps is to combine the process of making UBM pads and the process of making bumps in one patterning operation. As shown in FIG. 1A, initially a UBM layer 120 is formed over an entire active surface of a wafer 110 by a sputtering or other deposition method. The UBM layer 120 also covers the bonding pads 111 in addition to the passivation layer 112 of the wafer 110. Then, a dry film 130 is attached to the upper surface of the UBM layer 120. By means of a photomask 150, the dry film 130 is exposed and developed to form a plurality of openings 131 in positions corresponding to the bonding pads 111. Because the dry film 130 is required to have a thickness thick enough for forming bumps 140, there are some uncontrollable reentrant portions 132 formed at the bottom edge of the openings 131 while exposing and developing the dry film 130. As shown in FIG. 1B, a plurality of solder bumps 140 are formed in the openings 131 also formed in the reentrant portions 132 by plating. After the step of exposing and developing the dry film 130, the reentrant portions 132 in the openings 131 with random sizes and shapes are formed at the bottom of the openings 131, so that the bumps 140 have uncontrollably and unequally footprints on the UMB layer 120, i.e. the covering areas that the bumps 140 cover the UMB layer 120 are expanded and difficult to keep in uniform size. Therefore a plurality of UBM pads 121 are formed with uncontrollable areas after etching the UBM layer 120 under the mask of bumps 140. Generally, the thicker the dry film 130 is, the worse precision of the sizes of the UBM pads 121 becomes. As shown in FIGS. 1C and 1D, the reflowed bumps 140 formed by reflowing the bumps 140 wet the corresponding UBM pads 121 to become spherical shapes without equal height. Therefore, the shapes of UBM pads 121 seriously affect the heights of the reflowed bumps 140.